Fructose, or grape sugar, has been used as a sugar substitute in many applications. Increasing sugar prices spur increased fructose usage, and current fructose production is at an all-time peak.
The largest commercial source of fructose is glucose, an end product of starch hydrolysis, which is isomerized to fructose by enzymatic methods using glucose isomerase. Because the high enzyme cost dictates its reuse, the isomerization is not done homogeneously where recovery of enzyme would be difficult and costly, but instead is performed heterogeneously using glucose isomerase immobilized in some manner. That is, either cells containing glucose isomerase, or the enzyme itself, is physically and/or chemically bound to a support through which flows a glucose feedstock, with isomerization of glucose to fructose attending contact of the feedstock with the immobilized glucose isomerase system (IMGI).
It is highly desirable to have the IMGI as productive as possible. A measure of its productivity is its half-life, by which is meant the time necessary to reduce the activity of an IMGI to one-half its initial value. We have repeatedly observed that the half-life of an IMGI was longer using a purified glucose feedstock than with a commercial feedstock. These observations led to the discovery that the deleterious effect on IMGI half-life was directly associated with the presence of carbonyl-containing components in the feedstock which formed a precipitate with 2,4-dinitrophenylhydrazine. It was further discovered that removal of these components, or poisons, from the feedstock led to a substantial increase in the half-life, and thus the productivity, of IMGI. A crucial discovery leading to the invention described herein is that certain reducing agents very effectively chemically alter these poisons, or deleterious components, in a glucose feedstock, thereby rendering them innocuous and, in effect, removing the poisons initially present.